1. Field of the Invention
The present invention relates to vapor compression systems and, more particularly, to a transcritical vapor compression system in which the efficiency and capacity of the system can be adjusted.
2. Description of the Related Art
Vapor compression systems are used in a variety of applications including heat pump, air conditioning, and refrigeration systems. Such systems typically employ working fluids, or refrigerants, that remain below their critical pressure throughout the entire vapor compression cycle. Some vapor compression systems, however, such as those employing carbon dioxide as the refrigerant, typically operate as transcritical systems wherein the refrigerant is compressed to a pressure exceeding its critical pressure and wherein the suction pressure of the refrigerant is less than the critical pressure of the refrigerant, i.e., is a subcritical pressure. The basic structure of such a system includes a compressor for compressing the refrigerant to a pressure that exceeds its critical pressure. Heat is then removed from the refrigerant in a first heat exchanger, e.g., a gas cooler. The pressure of the refrigerant exiting the gas cooler is reduced in an expansion device and the refrigerant then absorbs thermal energy in a second heat exchanger, e.g., an evaporator, before being returned to the compressor. The first heat exchanger of such a system can be used for heating purposes, alternatively, the second heat exchanger can be used for cooling purposes.
FIG. 1 illustrates a typical transcritical vapor compression system 10. In the illustrated example, a two stage compressor is employed having a first compression mechanism 12 and a second compression mechanism 14. The first compression mechanism compresses the refrigerant from a suction pressure to an intermediate pressure. An intercooler 16 is positioned between the first and second compression mechanisms and cools the intermediate pressure refrigerant. The second compression mechanism then compresses the refrigerant from the intermediate pressure to a discharge pressure that exceeds the critical pressure of the refrigerant. The refrigerant is then cooled in a gas cooler 18. In the illustrated example, a suction line heat exchanger 20 further cools the high pressure refrigerant before the pressure of the refrigerant is reduced by expansion device 22. The refrigerant then enters evaporator 24 where it is boiled and cools a secondary medium, such as air, that may be used, for example, to cool a refrigerated cabinet. The refrigerant discharged from the evaporator 24 passes through the suction line heat exchanger 20 where it absorbs thermal energy from the high pressure refrigerant before entering the first compression mechanism 12 to repeat the cycle.
The capacity and efficiency of such a transcritical system can be regulated by regulating the pressure of the refrigerant in gas cooler 18. The pressure of the high side gas cooler may, in turn, be regulated by regulating the mass of refrigerant contained therein which is dependent upon, among other things, the total charge of refrigerant actively circulating through the system. It is known to provide a reservoir in communication with the system for retaining a variable mass of refrigerant. The total charge of refrigerant actively circulating through the system can then be adjusted by changing the mass of refrigerant contained within the reservoir. By regulating the mass of refrigerant actively circulated through the system, the pressure of the refrigerant in the gas cooler can also be regulated. One problem associated with use of such reservoirs to contain a variable mass of refrigerant is that they can increase the cost and complexity of the system.
An alternative apparatus and method for adjusting the efficiency and capacity of a transcritical vapor compression system is desirable.